The present invention relates to a measuring amplifier of the type including an operational amplifier provided with a feedback-connected element which determines the amplifier measuring range.
The feedback-connected element may be, for example, a resistor which defines a linear measuring range. The higher its resistance, the more sensitive is the measuring amplifier.
Another element in the feedback branch of the measuring amplifier may be, for example, a diode which is basically a resistor with a logarithmic characteristic. It causes the measuring amplifier to have a logarithmic measuring range. It is known, for example, to provide one diode and a plurality of different resistors each of which can be selectively connected in the feedback branch with the aid of a measuring range switch.
While linear measuring ranges are used predominantly for measuring purposes because of the arbitrarily selectable sensitivity graduation, i.e. up to 10.sup.-12 A, and the high accuracy, i.e. about 1%, the logarithmic measuring range, which usually extends across all of the linear ranges, serves for control and regulating purposes. In many cases it is then desired to have, for example, the measured signal and the regulating signal available at the same time. This requirement becomes the more difficult to satisfy as more stringent performance requirements are placed on the measuring amplifier, for example with respect to sensitivity, measuring accuracy and number of decades in the logarithmic measuring range. In the case of measuring amplifiers capable of measuring currents down into the pA range, with permissible measuring accuracies in the linear ranges of about 1%, and with a logarithmic range of 6 to 8 decades, it has thus far been impossible to obtain simultaneous measurements in the logarithmic and linear scales. A compromise solution is the use of two measuring instruments with two separate sensors as the current supply sources. However, it has not been possible to realize this in a practical manner and was thus impossible to have two signals available simultaneously. If the compromise solution could be realized, it would entail very high expenses.